This invention is related to nondestructive testing methods and, more particularly, to acoustic methods for evaluating the fatigue life of materials.
The development of increasingly higher performance requirements and the need to carefully control building costs have frequently led to the design of modern structures in accordance with a "safe life" or "damage tolerant" philosophy. In this approach, structural components are specified with dimensions calculated so that fatigue cracks and resulting damage will not progress to a catastrophic level prior to detection at scheduled inspection periods. This design procedure recognizes that no part is likely to be perfect or remain so during its intended lifetime of use.
Thus, under modern design practices, a flaw of a subcritical size must be assumed to be present in the structure. The upper limit of this flaw's size is determined by the sensitivity of the inspection system to be used, at a 100% confidence level for detection of the flaw. At the present time, the sensitivity of nondestructive evaluation (NDE) methods recognized in the art, such as the dye penetrant, magnetic particle, ultrasonic, eddy current, and radiographic inspection methods, is generally considered to be approximately 1 mm. Thus, for example, if it is assumed that no surface flaw larger than 1 mm has escaped detection during inspection, and if it is further assumed that a very simple tension-tension fatigue load of constant amplitude is applied to the structure, the remaining life of the part, or the minimum number of remaining cycles to failure .DELTA.N, can be estimated by interpretation of the "Paris equation": EQU da/dN=A(.DELTA.K).sup.m ( 1)
where da/dN is the rate of crack growth, .DELTA.K is the stress intensity range, and A and m are constants for a particular material. Further manipulation of this expression, together with some simple assumptions, can be performed to derive an expression for the critical material dependent flaw size. Table I lists some estimated critical flaw sizes for several important structural materials.
TABLE I. ______________________________________ Order of Magnitude Estimates of Critical Flaw Sizes in Some Structural Alloy. Materials Critical Flaw Size (mm) ______________________________________ Steels 4340 1.5 D6AC 2.3 Marage 250 4.2 9Ni4Co 20C 27.5 Aluminum 2014-T651 8.0 Alloys 2024-T3 27.5 Titanium 6A1-4V 8.0 Alloys ______________________________________
It is a basic goal of all major nondestructive evaluation programs to determine the size, shape, and orientation of subcritical defects, with all three parameters being equal in importance. As can be seen from Table I, however, some of the critical flaw sizes, even for metals, are quite small. Since the sensitivity of standard NDE techniques is limited to no less than approximately 1 mm, it is apparent that in some materials, a flaw may approach the critical size before it can be effectively detected by present NDE methods.
Consequently, a need has developed in the art for an NDE technique which is capable of detecting flaws or fatigue damage with a much higher sensitivity, i.e, much earlier in the fatigue life of an object, than is possible with known techniques. Critical components whose life is controlled by fatigue, for example, are at the present time considered in some structural applications to have failed as soon as the probability of forming a crack of a small but finite size is 0.1%. Regardless of whether or not such a crack actually exists in a particular component, the component is retired automatically. With a more precise NDE technique, however, each such component could be inspected upon reaching the 0.1% probability level, and only those particular parts whose inspection revealed unacceptably large cracks would be retired. The latter approach, known as "retirement for cause", would be much more economical, but could provide the same level of reliability as prior art techniques.